1. Technical Field
The present invention relates generally to optical communication networks, and, more specifically, to optical transport networks as designated by the International Telecommunication Union.
2. Background
Optical networks are connected through optical fibers with elements capable of providing optical channel transport, multiplexing, routing, management of the network, supervision, and redundancy for survivability. Many telecommunications and data carriers around the world are increasingly using Optical Transport Networks (OTN) for their long-haul and metro-area networks. OTN is growing faster than Synchronous Optical Networking and Synchronous Digital Hierarchy (SONET/SDH) and has the potential of boosting bandwidth and increasing networking functionality.
Optical networks utilize optical fibers and lasers or highly coherent light from light-emitting diodes to transfer multiple digital bit streams of data over the network. SONET/SDH was originally designed to replace Plesiochronous Digital Hierarchy (PDH) which was used to transport large amounts of telephone calls and data traffic over the same fiber without requiring synchronization across the multiplexed tributary channels. PDH used circuit-switching and was spectrally efficient only if the sources of the tributary channels were synchronized. However, as these optical networks continued to grow, so did the traffic on them. SONET and SDH, a superset of SONET, were developed to support real-time, uncompressed, circuit-switched, digitally encoded voice and data. SONET/SDH allows for simultaneously transporting many different circuits (tributaries) of differing origin using a single framing protocol, and is ideal for transporting Asynchronous Transfer Mode (ATM) frames, Internet protocol (IP) packets, or Ethernet frames. Generally, a frame is a group of data bits in a specific format (ATM, Ethernet, IP and others) with a flag at the beginning and the end of the data bits to define the individual frame.
The message protocols transported by SONET and SDH are similar with a few exceptions. SONET is typically used in North America whereas SDH is widely used throughout the world. The protocol of SONET/SDH is a time (byte) division multiplexed structure wherein a header is interleaved between the data to permit the encapsulated data to have its own unique frame rate and be present within the SONET/SDH frame structure and rate. The protocols buffer data during transit for at least one frame before sending. This buffering allows for multiplexed data to move within the overall framing (transmission) to compensate for different frame rates. The protocol becomes more complex based on when and where in the data stream padding is needed and at what level of the multiplexing structure.
In optical networks, SONET/SDH add/drop multiplexers and cross-connectors have relatively high power consumption, and with increased demand for these networks for industry, public works, school, and residential use, energy usage increases. The networking community's energy saving object is becoming more important now that Internet traffic is expected to continue steep growth driven by video applications and cloud computing advances.
Energy consumption is a consideration in designing communication networks and subsystems including hardware, add/drop multiplexers, and cross-connectors. For example, Internet Protocol (IP) routers can lower their packet processing rate when traffic volume is low to reduce energy consumption in both optical and electrical networks. All-optical Wavelength Division Multiplexing (WDM) networks can be made more energy efficient by bypassing the optical-electrical-optical conversion at the intermediate optical cross-connection nodes. One layer of the communication networks where increased energy efficiency is desirable in current and future networks is in the third network layer, the OTN. The OTN layer is often used between the IP and the WDM layer to provide sub-wavelength capacity to the links of routers. Present day OTN solutions perform similar to SONET/SDH and perform digital time division multiplexing of multiple sub-wavelength channels to fill out the entire wavelength of a channel. Each sub-wavelength channel is individually routed using Digital Cross-Connects (DXC), and each DXC requires approximately 10 Watts per 10 Gigabits of carried data to perform transport functionalities using current technology. Similar levels of energy consumption take place in the OTN add/drop multiplexers, where tributary signals from the end-user are injected into or extracted from the OTN network. As the energy consumption of telecommunications networks is forecast to grow manifold due to the rapid increase of traffic volume in broadband networks, combined with the expectation of higher energy prices and increasing concerns about global warming, finding energy-efficient solutions becomes an important issue for telecommunications networks.
At the IP layer, energy-aware packet forwarding techniques suggest that smaller IP packets increase the energy consumption of routers, so optimizing the size of IP packets can make routers more energy efficient. However, reducing switching delay and lowering energy consumption need to be carefully balanced. New network architecture comprising two parallel networks have been proposed. A “super-highway” network using pipeline forwarding for IP packets would be used in conjunction with the current Internet which carries traditional traffic and signaling between routers that set up synchronous pipes in super-highway networks. The super-highway would carry traffic that has predictable patterns and require high bandwidth.
In WDM networks, high energy consumption originates from the optical network equipment which is used for traffic grooming. Hence, energy-efficient traffic grooming, which reduces the number of required lightpaths, considerably increases energy savings. Other approaches to reduce energy consumption include using Routing and Wavelength Assignment (RWA) heuristics that minimize the number of lightpath interfaces, and using digital signal processing for wavelength translation of the optical frequencies of each specific wavelength on the optical fiber when required. However, this process may be cost prohibitive due to the expense of the optical equipment based on existing technology needed to create the wavelength translation. Other possible solutions include reducing energy consumption of each network operation by performing dynamic traffic grooming over time.
Current telecommunications networks are based on an architectural model involving three classes of network domains: core, metro, and access. In core networks, efforts to reduce energy consumption can be divided into two categories: energy-efficient network design and energy-efficient network operations. The energy consumption of IP routers, EDFAs, and transponders is jointly minimized for an IP-over-WDM network by utilizing Mixed Line Rates (MLR). Likewise, shutting down idle network elements saves energy. To identify the maximum number of idle nodes and links while still supporting the ongoing traffic, heuristics and Mixed Integer Linear Program (MILP) models can be used to reduce the powered nodes (or equipment) during off-peak hours and during traffic fluctuations throughout the day. Similarly, idle line cards can be shut down when traffic load is low, while keeping the physical topology invariant or with the minimum required level of connectivity, to reduce power needs. “Green Routing” has been proposed which uses energy consumption of network equipment as the optimization objective. Also, greater attention is being paid to renewable energy. One idea to reduce carbon footprint is to establish core servers, switches, and data centers at locations where renewable energy can be found, and then to route traffic to the “green areas”.
Wireless-Optical Broadband Access Network (WOBAN) is a novel access architecture, and can provide high-bandwidth services. Energy savings in the optical part of WOBAN by sleeping mechanism has been studied, and energy-efficient design of a unidirectional WDM ring network has been investigated.
Energy-efficiency is a major problem for data centers, which are vital to support current data applications. Optical networks play an important role in both data center inter- and intra-connections. An approach to reducing the energy consumption of high-speed intra-connection (inside data centers) links has been. Load distribution across data centers in different locations is also related with power-conservation. How to optimally distribute requests has also been studied.
Solutions based on analog Frequency Division Multiplexing (FDM) were widely used in the pre-SONET/SDH era, to multiplex transport channels together using spectral diversity in coaxial and fiber cables. Although analog RF/microwave Subcarrier Multiplexing (SCM) in fiber optics is still in use to carry radio signals between antennas and base station, FDM technologies in digital transport networks were abandoned due in part to their relatively low spectral efficiency when compared to Time Division Multiplexing (TDM) and synchronous transmission techniques, such as SONET and SDH. Another problem of traditional FDM (or SCM), being analog systems, is their susceptibility to accumulated waveform distortion and crosstalk. For these reasons analog FDM is not a competitive solution for large-scale optical networks. As an extension of SCM, Orthogonal Frequency Division Multiplexing (OFDM) introduces orthogonality between adjacent subcarrier channels, so that no guard band is required between adjacent channels, which maximizes optical bandwidth efficiency.